This invention relates to processing of semiconductor wafers and, more particularly, relates to a wafer handling system including apparatus for providing individual wafers with a preselected orientation relative to a treatment beam.
Ion implantation has become a standard technique for introducing impurities into semiconductor wafers in a controlled and rapid manner. A beam of ions is generated in a source and directed with varying degrees of acceleration toward the semiconductor wafer. The impurities are introduced into the bulk of semiconductor wafers by using the momentum of the ions as a means of embedding them in the crystalline lattice of the semiconductor material. In bombarding the semiconductor material, it is desirable to carefully control the depth of penetration of the impurity ions so as to control the characteristics of the semiconductor device being fabricated. One factor which determines depth of penetration is the kinetic energy of the impinging ions.
Another factor determinative of depth of penetration is the angle at which the ion beam impinges on the crystalline structure of the semiconductor material, typically silicon. For certain angles of incidence of the ion beam upon the crystalline lattice, it is known that an increase in depth of penetration, referred to as channeling, occurs. The channeling effect arises because rows or planes of atoms can steer energetic ions by means of a correlated series of gentle, small-angle collisions. Since silicon semiconductor wafers are typically cut with a particular crystal axis normal to the surface, it has been the practice in ion implantation to tilt the wafer with respect to the ion beam to avoid channeling effects. Typically, the wafer is tilted so as to provide an angle of about 7 degrees between the ion beam and the normal to the wafer. However, when the wafer is tilted, channeling occurs for certain angles of rotation of the wafer due to rotation of various crystal planes into alignment with the incident ion beam. See, for example, Chu et al, "Backscattering Spectrometry," Academic Press, New York, 1978, pp. 227-228. Therefore, in order to have complete control over the depth of penetration of implanted ions, it is necessary not only to tilt the semiconductor wafer but also to provide a rotational orientation for which channeling is minimal.
One of the major objectives in commercial semiconductor processing is to achieve a high throughput in terms of wafers processed per unit time. To assist in achieving high throughput, automated wafer transfer systems have been developed. These systems typically transfer wafers from a wafer carrier, or cassette, into a wafer processing chamber and then back into the cassette without intervention by an operator. See, for example, U.S. Pat. No. 4,311,427, issued Jan. 19, 1982, to Coad et al, which discloses an automated wafer transfer system for a sputtering system. The wafers in the cassette normally have an unknown rotational orientation when the cassette is introduced into the system. It is therefore desirable to provide such wafer transfer systems with the capability of rotating the wafers into a preselected orientation prior to processing in order to avoid the above-described effects of channeling. It is further desirable that an apparatus for orienting wafers should be simple, low cost, accurate, and fast acting in order to avoid an adverse impact upon the speed and cost of processing wafers.
Apparatus for wafer orientation have been shown in the prior art. U.S. Pat. No. 4,311,427 discloses the use of rotating rollers for aligning the guide flats of wafers in a cassette. U.S. Pat. No. 3,901,183, issued Aug. 26, 1975, to Wittkower, discloses the use of a rotating disk on a wafer holder in a vacuum chamber for aligning the guide flat of a wafer. However, such systems can only align the flat at one predetermined angle. In some wafer processing steps, this limitation is undesirable. The wafer guide flat is not always indicative of the same semiconductor crystal plane. Therefore, simple flat alignment does not always produce the same angle between the crystal lattice and an ion beam. To provide flexibility, the wafer handling system should be programmable to permit any preselected orientation of the wafer.
A separate wafer orientation station can be provided between the cassette and the wafer processing chamber. However, such an arrangement adds complexity and cost to the wafer transfer system. Furthermore, the speed of the wafer transfer system is reduced and the risk of wafer damage or breakage is increased when an additional wafer transfer step is introduced. Finally, errors in wafer orientation can arise as the wafer is transferred from the orientation station to the processing chamber.
It is a general object of the present invention to provide apparatus for orienting a semiconductor wafer prior to processing in a wafer processing chamber.
It is another object of the present invention to provide a simple apparatus for rotating a semiconductor wafer through any preselected angular displacement.
It is yet another object of the present invention to provide apparatus for orienting a semiconductor wafer in an automated wafer transfer system.
It is still another object of the present invention to provide apparatus for orienting a semiconductor wafer at an entrance to a semiconductor processing chamber.